1. Field of the Invention
This invention relates to solvent-free or low-solvent coating compositions which harden to form polyurethanes and to the use thereof for the coating of substrates, in particular textiles.
2. Description of the Prior Art
The coating of textiles with polyurethane urea solutions containing up to 70%, by weight, of solvents is known. Either completely reacted polyurethane ureas or those which require a second component for subsequent crosslinking in order to achieve the coating properties thereof may be used for this purpose. Coating compositions of the last-mentioned type have been described, for example, in U.S. Pat. No. 3,711,571, according to which the dissolved polyurethane ureas are mixed with oxime-blocked isocyanate prepolymers and cured during a heat treatment in a drying channel. This method of coating has the disadvantage of requiring at least 50%, by weight, of solvents (not counting the quantity of blocking agent).
Other polyurethane urea coating compositions, which do not require solvent, have been disclosed in U.S. Pat. No. 3,755,261 and in German Offenlegungsschrift No. 2,462,317. These are thermosetting mixtures of liquid isocyanate prepolymers and a latent hardener based on salts of 4,4'-diaminodiphenylmethane ("MDA"). The latent hardener may be liquefied by suspending the MDA salt complex in at least 50%, by weight, of a PVC plasticizer of the phthalic acid ester type. According to the examples given, the isocyanate prepolymers contain a relatively high proportion of free diisocyanates.
Such a coating system has disadvantages in the physiological field. The relatively high free diisocyanate content and particularly the toxicity of MDA used as a constituent of the latent hardener are problematic. A further disadvantage of such coatings is the plasticizer content thereof. Due to "blooming" or "bleeding" of the plasticizer, dry top coats having perfect surfaces are unobtainable.
In British Pat. No. 970,459, there is described a process for bonding a foam foil to a textile substrate. The adhesive coating used is a reactive mixture consisting substantially of ketoxime-blocked isocyanate prepolymer and N,N,N',N'-tetrakis-(2-hydroxypropyl)-ethylene diamine as crosslinking agent. Since this system is cured via the urethane groups and not via urea groups, the end products are soft films which are not suitable for the production of top coats.
According to publication by J. Verhanik (publication of lectures held at the Congress of the Leather Industry, 18-22.10.78 in Budapest, pages 1279 to 1288, OMKDK-Technoinform 1428; Budapest Postbox 12) concerning a "thermoactive, solvent-free PUR system", polyether based isocyanate prepolymers which are blocked with phenol groups may be worked-up with polyether amines having molecular weights of approximately 750 to produce foam coatings. Apart from the toxic effect of the phenol blocking agent when released, this system has the disadvantage that it is restricted to a very narrow field of application on account of its mechanical properties.
It was, therefore, an object of the present invention to overcome the disadvantages mentioned above and provide a solvent-free or low solvent, physiologically harmless coating system which would fulfill the technical requirements of any coating component used, for example, for an adhesive coat, foam coat or top coat, including also a fusible top coat which may be welded.
This problem could be solved by using the coating composition described below.